The insulation packages are normally attached in the area of the outer metal skin of the aircraft. They represent primary insulation, while in many cases an additional secondary insulation in the form of insulation packages is applied directly to the cabin lining.
For safety reasons and for easier handling, the insulation packages consist of non-combustible and/or hard-to-ignite mats of insulating material (mostly of mineral wool) which, as with a cushion, are enclosed by an encapsulating membrane.
At cruising altitude, surfaces temperatures of −30° C. and below prevail on the outer metal skin of the aircraft. This is the equivalent of a saturation vapor pressure of less than 40 Pa. The air in the cabin is at a relative humidity of about 15%, even without any comfort-related air humidification. At a cabin temperature of 23° C., this is the equivalent of a vapor pressure of about 400 Pa, ten times the saturation vapor pressure at the outer skin. As a result of the pressure differential, a diffusion flow into the insulation packages in the primary insulation is created if the cabin air can penetrate behind the lining through leaks, which is normally the case. The insulating material in the insulation packages slowly becomes more moist during the flight as a result of the resulting condensate. Once the aircraft is on the ground, the outer skin warms up and the insulation packages can dry out again to a limited extent. In order to prevent excessive entry of moisture during the flight, the conventional encapsulating membrane in the insulation package has a fixed vapor diffusion resistance of about Im of diffusion-equivalent air layer thickness (sd value). An encapsulating membrane of this type results in increased moisture in the insulation packages, which can lead to additional energy-inefficient weight and an increase in fuel consumption as well as condensate dripping from the cabin ceiling. The increase in cabin air humidity planned for the future to improve the physiological air quality in aircraft will, in all probability, further increase the condensate problems in the insulation packages.